The invention concerns anti-scatter grids as used in radiology imaging and particularly in X-ray imaging.
A radiology imaging apparatus conventionally comprises a source of radiation, such as an X-ray source, and a means for forming the image, such as an image receptor, between which the object to be imaged is positioned. The beam of radiation emitted by the source passes through the object before reaching the receptor. It is partly absorbed by the inner structure of the object so that the intensity of the beam received by the receptor is attenuated. The global attenuation of the beam after passing through the object is directly related to absorption distribution within the object.
The image receptor comprises an optoelectronic detector or intensifying screen-film couple, sensitive to radiation intensity. Consequently, the image generated by the receptor corresponds in principle to the distribution of global ray attenuation subsequent to passing through inner structures of the object.
Part of the radiation emitted by the source is absorbed by the inner structure of the object, the other part is either transmitted (primary or direct radiation) or scattered (secondary or scatter radiation). The presence of scatter radiation leads to degradation of contrast in the image obtained and a reduced signal to noise ratio. This is of particular hindrance, in particular if it is desired to visualize details of the object.
One solution to this problem comprises inserting an “anti-scatter” grid between the object to be X-rayed and the image receptor. These grids are usually formed of a series of parallel strips or partitions of X-ray absorbing material. In so-called “focalized” grids (according to the terminology laid down by standard IEC 60627 on “X-ray imaging diagnostic equipment—Characteristics of anti-scatter grids for general use and mammography screening”) all the planes of the strips or partitions are oriented along planes passing through the focal point of radiation emitted by the source. Therefore, these grids allow direct radiation to pass and absorb scatter radiation. Focalized anti-scatter grids have contributed towards a considerable improvement in the contrast of images obtained.
In order to obtain good quality images it is desirable to provide grids having the finest possible structure so as not to disturb direct radiation. It is also desirable to control the orientation of the absorbing strips or partitions with precision. The precision with which the strips or partitions are orientated evidently depends upon the manufacturing technique used to produce the grid. However, it is found that during use of the grid it may undergo deformation, which substantially modifies strip orientation. The consequence is impaired precision of strip or partition orientation. This impairment is greater the narrower the thickness of the grid and its propensity to deform.
This problem is particularly raised in imaging devices with an overhanging grid, i.e., fixed on one side only. In this case it may undergo substantial bending stresses.
To overcome these disadvantages, grids have been proposed having an aluminium frame, the frame giving rigidity to the assembly. In addition, these grids are coated on each of their surfaces with plates in a composite carbon and resin material having a thickness of between 0.2 and 0.4 mm.